The present invention relates generally to seals that interact with lubricant during rotation of a relatively rotatable surface to wedge a film of lubricant into the interface between the seal and the relatively rotatable surface to reduce wear. More specifically, the present invention concerns the provision of static and dynamic sealing lips in a hydrodynamic seal that controls interfacial contact pressure within the dynamic sealing interface for efficient hydrodynamic lubrication and environmental exclusion while permitting relatively high initial compression and relatively low torque.
FIG. 1 of this specification represents a commercial embodiment of the prior art of U.S. Pat. No. 4,610,319, and FIG. 1A represents a commercial embodiment of the prior art of U.S. Pat. No. 5,678,829. These figures are discussed herein to enhance the readers"" understanding of the distinction between prior art hydrodynamic seals and the present invention. The lubrication and exclusion principles of FIG. 1 are also used in the prior art seals of U.S. Pat. Nos. 5,230,520, 5,738,358, 5,873,576, 6,036,192, 6,109,618 and 6,120,036, which are commonly assigned herewith. The aforementioned patents pertain to various seal products of Kalsi Engineering, Inc. of Sugar Land, Tex. that are known in the industry by the registered trademark xe2x80x9cKalsi Sealsxe2x80x9d, and are employed in diverse rotary applications to provide lubricant retention and contaminant exclusion in harsh environments.
Referring now to FIG. 1, the seal incorporates a seal body 18 that is solid and generally ring-like, and defines a lubricant end 20 and an environment end 22. The seal incorporates a dynamic sealing lip 24 defining a dynamic sealing surface 26 and also defining a exclusionary geometry 28 which may be abrupt, and which is for providing environmental exclusion.
The dynamic sealing lip 24 has an angulated flank 30 having intersection with the seal body at lip termination point 32. Angulated flank 30 is non-circular, and forms a wave pattern about the circumference of the seal, causing dynamic sealing surface 26 to vary in width.
Hydrodynamic inlet radius 38 is a longitudinally oriented radius that is the same size everywhere around the circumference of the seal, and is tangent to dynamic sealing surface 26 and angulated flank 30. Since hydrodynamic inlet radius 38 is tangent to angulated flank 30, it also varies in position about the circumference of the seal in a wavy manner. Angulated flank 30 defines a flank angle 40 that remains constant about the circumference of the seal. The tangency location 42 between hydrodynamic inlet radius 38 and dynamic sealing surface 26 is illustrated with a dashed line.
When installed, the seal is located within a housing groove and compressed against a relatively rotatable surface to establish sealing contact therewith, and is used to retain a lubricant and to exclude an environment. When relative rotation occurs, the seal remains stationary with respect to the housing groove, maintaining a static sealing relationship therewith, while the interface between the dynamic sealing lip 24 and the mating relatively rotatable surface becomes a dynamic sealing interface. The lubricant side of dynamic sealing lip 24 develops a converging relationship with the relatively rotatable surface a result of the compressed shape of hydrodynamic inlet radius 38.
In response to relative rotation between the seal and the relatively rotatable surface, the dynamic sealing lip 24 generates a hydrodynamic wedging action that introduces a lubricant film between dynamic sealing lip 24 and the relatively rotatable surface.
The compression of the seal against a relatively rotatable surface results in compressive interfacial contact pressure that is determined primarily by the modulus of the material the seal is made from, the amount of compression, and the shape of the seal. The magnitude and distribution of the interfacial contact pressure is one of the most important factors relating to hydrodynamic and exclusionary performance of the seal.
The prior art seals are best suited for applications where the pressure of the lubricant is higher than the pressure of the environment. Owing to the complimentary shapes of the seal environment end 22 and the mating environment-side gland wall, the seal is well supported by the environment-side gland wall in a manner that resists distortion and extrusion of the seal when the pressure of the lubricant is higher than the pressure of environment.
If the pressure of the environment is substantially higher than the pressure of the lubricant, the resulting differential pressure-induced hydrostatic force can severely distort body 18, hydrodynamic inlet radius 38 and exclusionary geometry 28. The hydrostatic force presses body 18 against the lubricant-side gland wall, and can cause body 18 to twist and deform such that angulated flank 30 and hydrodynamic inlet radius 38 are substantially flattened against the relatively rotatable surface. Such distortion and flatting can inhibit or eliminate the intended hydrodynamic lubrication, resulting in premature seal wear because the gently converging relationship between dynamic sealing lip 24 and the relatively rotatable surface (which is necessary for hydrodynamic lubrication) can be eliminated. Such distortion can also cause exclusionary geometry 28 to distort to a non-circular configuration and may also cause portions of dynamic sealing surface 26 to lift away from the relatively rotatable surface, producing a low convergence angle between dynamic sealing surface 26 and the relatively rotatable surface on the environment edge, and causing the exclusionary geometry 28 to become non-circular and skewed relative to rotational velocity V. Such distorted geometry is eminently suitable for the generation of a hydrodynamic wedging action in response to relative rotation of the relatively rotatable surface. Such wedging action can force environmental contaminants into the sealing interface and cause rapid wear.
To effectively exclude a highly pressurized environment, one must use a pair of oppositely-facing prior art hydrodynamic seals; one to serve as a partition between the lubricant and the environment, and the other to retain the lubricant, which must be maintained at a pressure equal to or higher than the environment. This scheme ensures that neither seal is exposed to a high differential pressure acting from the wrong side, but requires a mechanism to maintain the lubricant pressure at or above the environment pressure. For example, see the sealed chambers of the artificial lift pump rod seal cartridge of U.S. Pat. No. 5,823,541, and see the first pressure stage of the drilling swivel of U.S. Pat. No. 6,007,105.
Many applications, such as the oilfield drilling swivel, the progressing cavity artificial lift pump, centrifugal pumps, and rotary mining equipment would benefit significantly from a hydrodynamic rotary seal having the ability to operate under conditions where the environment pressure is higher than the lubricant pressure. The resulting assemblies would avoid the complexity and expense associated with using pairs of seals having lubricant pressurization there-between.
In the absence of lubricant pressure, the compressed shape of the environment end 22 becomes increasingly concave with increasing compression because the compression is concentrated at one end of the seal. This reduces interfacial contact pressure near exclusionary geometry 28 and detracts from its exclusionary performance. In the presence of differential pressure acting from the lubricant side of the seal, the environment end 22 is pressed flat against the wall of the housing groove, which increases the interfacial contact pressure near exclusionary geometry 28 and improves exclusionary performance.
Although such seals perform well in many applications, there are others where increased lubricant film thickness is desired to provide lower torque and heat generation and permit the use of higher speeds and thinner lubricants. U.S. Pat. No. 6,109,618 is directed at providing a thicker film and lower torque, but the preferred, commercially successful embodiments only work in one direction of rotation, and are not suitable for applications having long periods of reversing rotation.
Interfacial contact pressure at hydrodynamic inlet radius 38 tends to be relatively high, which is not optimum from a hydrodynamic lubrication standpoint, and therefore from a running torque and heat generation standpoint. Hydrodynamic inlet radius 38 is relatively small, and therefore the effective hydrodynamic wedging angle developed with the relatively rotatable surface is relatively steep and inefficient.
Running torque is related to lubricant shearing action and asperity contact in the dynamic sealing interface. Although the prior art hydrodynamic seals run much cooler than non-hydrodynamic seals, torque-related heat generation is still a critical consideration. The prior art seals are typically made from elastomers, which are subject to accelerated degradation at elevated temperature. For example, media resistance problems, gas permeation problems, swelling, compression set, and pressure related extrusion damage all become worse at higher temperatures. The prior art seals cannot be used in some high speed or high-pressure applications simply because the heat generated by the seals exceeds the useful temperature range of the seal material.
In many of the prior art seals, interfacial contact pressure decreases toward exclusionary geometry 28, and varies in time with variations in the width of the interfacial contact footprint. Neither effect is considered optimum for exclusion purposes. When environmental contaminant matter enters the dynamic sealing interface, wear occurs to the seal and the relatively rotatable surface.
A certain minimum level of compression is required so that the seal can accommodate normal tolerances, misalignment, seal abrasion, and seal compression set without loosing sealing contact with the relatively rotatable surface. Seal life is ultimately limited by susceptibility to compression set and abrasion damage. Many applications would benefit from a hydrodynamic seal having the ability to operate with greater initial compression, to enable the seal to tolerate greater misalignment, tolerances, abrasion, and compression set.
Prior art seals can be subject to twisting within the housing groove. Such seals are relatively stable against clockwise twisting, and significantly less stable against counter-clockwise twisting, with the twist direction being visualized with respect to FIG. 1. Commonly assigned U.S. Pat. Nos. 5,230,520, 5,873,576 and 6,036,192 are directed at helping to minimize such counter-clockwise twisting.
When counter-clockwise twisting occurs, interfacial contact pressure increases near hydrodynamic inlet radius 38 and decreases near exclusionary geometry 28, which reduces exclusionary performance. Such twisting can also make the seal more prone to skewing within the housing groove.
U.S. Pat. No. 5,873,576 teaches that typical hydrodynamic seals can suffer skew-induced wear in the absence of differential pressure, resulting from xe2x80x9csnakingxe2x80x9d in the gland that is related to circumferential compression and thermal expansion. If this snaking/skewing is present during rotation, the seal sweeps the shaft, causing environmental media impingement against the seal. U.S. Pat. No. 5,873,576 describes the skew-induced impingement wear mechanism in detail, and describes the use of resilient spring projections to prevent skew. Testing has shown that the projections successfully prevent skew-induced wear in the absence of pressure, as was intended, and as such are an improvement over older designs. However, if the environmental pressure exceeds the lubricant pressure, the differential pressure can, in some embodiments, deform the seal in ways that are less favorable to environmental exclusion.
Referring now to the prior art illustration of FIG. 1A, there is shown a cross-sectional view of a prior art seal representative of the commercial embodiment of U.S. Pat. No. 5,678,829. Features in FIG. 1A that are represented by the same numbers as those in FIG. 1 have the same function as the features of FIG. 1. Solid lines represent the uninstalled cross-sectional condition of the seal, and dashed lines represent the installed cross-sectional condition; note the twisted installed condition.
An annular recess 49 defines flexible body lips 52 and 55, one of which incorporates the dynamic sealing surface 26, angulated flank 30, hydrodynamic inlet radius 38, and exclusionary geometry 28. The reduction of interfacial contact pressure near the circular exclusionary geometry 28 is particularly severe in such seals because of the hinging of the flexible body lips, which angularly displaces the dynamic sealing surface 26 and exclusionary geometry 28. This tends to xe2x80x9cprop upxe2x80x9d the exclusionary geometry 28 as shown, minimizing its effectiveness.
The present invention relates to generally circular rotary shaft seals suitable for bi-directional rotation that are used to partition a first fluid from an second fluid, and that exploit the first fluid as a lubricant to lubricate at a dynamic sealing interface. It is preferred that the first fluid be a liquid-type lubricant, however in some cases other fluids such as water or non-abrasive process fluid can be used for lubrication. The second fluid may be any type of fluid, such as a liquid or gaseous environment or a process media, or even a vacuum-type environment.
The seal of the present invention is positioned by a machine element such as a housing, and compressed against a relatively rotatable surface, initiating sealing therebetween. The machine element may define a circular seal groove for positioning the seal. When relative rotation occurs, the seal preferably maintains static sealing with the machine element, and the relatively rotatable surface slips with respect to the seal at a given rotational velocity. (Alternate embodiments are possible wherein the seal can slip with respect to both the machine element and the relatively rotatable surface.) The seal defines generally opposed first and second seal ends, and incorporates a dynamic sealing lip and a static sealing lip of generally circular configuration that are in generally opposed relation to one another to minimize compression-induced twisting of the seal cross-section. The dynamic sealing lip defines a sloping dynamic sealing surface of variable width and a hydrodynamic inlet curvature of variable position. The static sealing lip defines a sloping static sealing surface for establishing static sealed relationship with the machine element, and is in generally opposed relation to the sloping dynamic sealing surface.
The variation in position of the hydrodynamic inlet curvature may be sinusoidal, or any other suitable repetitive or non-repetitive pattern of variation. The hydrodynamic inlet curvature can consist of any type or combination of curve, such a radius, and portions of curves such as ellipses, sine waves, parabolas, cycloid curves, etc.
The sloping dynamic sealing surface and the variable position hydrodynamic inlet curvature deform when compressed into sealing engagement against the relatively rotatable surface to define a hydrodynamic wedging angle with respect to the relatively rotatable surface, and to define an interfacial contact footprint of generally circular configuration but varying in width, being non-circular on the first footprint edge due to the aforementioned variations. The non-circular (i.e. wavy) first footprint edge hydrodynamically wedges a lubricating film of the first fluid into the interfacial contact footprint in response to a component of the relative rotational velocity, causing it to migrate toward the second footprint edge. The first footprint edge is sometimes referred to as the xe2x80x9clubricant sidexe2x80x9d or xe2x80x9chydrodynamic edgexe2x80x9d, and the second footprint edge is sometimes referred to as the xe2x80x9cenvironment sidexe2x80x9d or xe2x80x9cexclusion edgexe2x80x9d. The number and amplitude of the waves at the first footprint edge can vary as desired. The relatively rotatable surface can take any suitable form, such as an externally or internally oriented cylindrical surface, or a substantially planar surface, without departing from the spirit or scope of the invention.
The seal provides a dynamic exclusionary intersection of abrupt form that provides the interfacial contact footprint with a second footprint edge, sometimes called the xe2x80x9cenvironment edgexe2x80x9d, that is substantially circular to prevent hydrodynamic wedging action and resist environmental exclusion. In the preferred embodiment, the dynamic exclusionary intersection is an intersection between the sloping dynamic sealing surface and the second seal end.
In the preferred embodiment, an energizer of a form common to the prior art having a modulus of elasticity different from the seal body, such as an elastomeric ring, a garter spring, a canted coil spring, or a cantilever spring, is provided to load the dynamic sealing lip against the relatively rotatable surface. In simplified embodiments, the energizer can be eliminated, such that the seal has one or more flexible lips, or such that the seal is solid and consists of a single material.
The second seal end is curved outward in a generally convex configuration in the uncompressed shape. When the seal is installed, the convex shape changes to a more straight configuration that helps to maintain contact pressure at the second edge of the interfacial contact footprint.
The generally circular body of the preferred seal embodiment defines a dynamic control surface and a static control surface near the first seal end that are in generally opposed relation to one another, and can react respectively against the relatively rotatable surface and the machine element to minimize undue twisting of the installed seal, which helps to maintain adequate interfacial contact pressure at the second footprint edge, thereby facilitating resistance to intrusion of abrasives that may be present in the second fluid.
The preferred seal cross-section defines a depth dimension from the sloping dynamic sealing surface to the sloping static sealing surface, and also defines a length dimension from the first seal end to second seal end. In the preferred embodiment of the present invention, the ratio of the length dimension divided by the depth dimension is preferred to be greater than 1.2 and ideally is in the range of about 1.4 to 1.6 to help minimize seal cross-sectional twisting.
The seal can be configured for dynamic sealing against a shaft, a bore, or a face. Simplified embodiments are possible wherein one or more features of the preferred embodiment are omitted.
It is one object of this invention to provide a hydrodynamic rotary seal having low torque and efficient exclusionary performance for reduced wear and heat generation. It is a further object to provide a seal that can operate with relatively high compression to better resist abrasives and tolerate runout, misalignment, tolerances, and compression set.
Another object is to compress a sloping dynamic sealing surface of a hydrodynamic seal against a relatively rotatable surface to establish an interfacial contact footprint, whereby more compression and interfacial contact pressure occurs at a second footprint edge, and less compression and interfacial contact pressure occurs at a first footprint edge.